copyright © 2004-2013 curt hill java looking at our first console application in eclipse

Copyright © 2004-2013 Curt Hill

JavaLooking at our first console

application in Eclipse


A moment on paradigms• Java is well suited for either the console or GUI

paradigm• A console program views the keyboard/screen

as nearly a single device– We both read and write to it using strings

• A GUI program has a WIMP interface:– Windows– Input – Menus– Pointer

• The console paradigm is easier for starters, but we may do both

Eclipse• Eclipse is the Interactive

Development Environment that we will use

• Originally developed by IBM• Donated to public domain• Now maintained by the Eclipse

Foundation– www.eclipse.org


http://www.eclipse.org/

Terminology for Eclipse• Resource

– Things such as projects, directories and files

• Editor– A window pane that allows changes to

a resource, often Java code

• View– A window pane that shows a resource

• Perspective– A group of editors and views



Eclipse Construction steps• Create new project• Create new class• Customize as needed• Debug and run


Create new project

• Click File|New|Java Project• Use the New Project dialog to set

some initial parameters • Specify the project name• This will become a directory tree• The project is an XML file of

name: .project in the directory• The screen shots follow

Create New Project


Enter Project Name


A usually skipped screen


Projects and Classes• A project is just a tool to manage

pieces• We now have to add the pieces• Each Java program is a class• Any class might use other classes

as well• What we now do is add the main or

root class to our project


Create new class• Click File|New|Class• This generates a new class

– This new class will be your program

• Use java.lang.Object as the ancestor class

• Specify a main method– Click on the box


Add an initial class


Name the class


Specified names• In the prior screen there were three

things that were specified:– Package– Class name– Inclusion of the main method via

checkbox

• The package name is usually all lower case

• The class name usually starts with a capital letter and each subsequent word is capitalized:FirstCon


Generated Code



Customize as needed

• At this point we have a Java console program with proper form, which does nothing– Now add the desired functionality

• Insert into main:System.out.println(“Hello world”);

• Java is case sensitive so System must start with a capital

• That doesn’t sound like much work– It will get worse

Customized



Debug and run• Click the run button or use menu

Run|Run• Run time properties are displayed

– Specify the new class name– Search may be easiest

• Console output will appear in the bottom pane– First the command line– Next the program output

Start Run


Run



package firstcon;public class FirstCon {/** * @param args */public static void main (String[] args) { // TODO Auto-generated method stub System.out.println("Hello World"); }}

Code


Comments• A comment explains things for

people– Ignored by Java

• /* and */– Enclose multiline comments

• // – Makes the rest of the line a comment

• Documentation uses special comments– Program: Javadoc


Class Declaration

• public class FirstCon {• Objects in Java are called Classes

– A class binds together variables and methods

• This class is named FirstCon• Its visibility is public

– A private class in this case could not be executed


Classes and files• A file is the compilation unit

– It must have an extension .java– Each file must have one and only one

public class

• The file name must match the public class name – Even case, which may be a problem

on MS Operating Systems

• Most of these issues are handled by Eclipse without our intervention


Compilation Units• Public class names must match file

name• Other private or friendly classes

may be present in the file• Each class will have its own object

(.CLASS) file after it is compiled


Main Function

• Most classes have both variables and methods

• Every application must have a method called main, which is where execution starts

• Main’s parameter is a string array – Command line parameters

• Often, in a console program most of the work occurs in the main method


Main function• public static void main(String

args[ ]) { System.out.println("Hello World!"); } // end of main

• void indicates main returns nothing• It is public

– Accessible to system


Main function• It is static

– A static function has limited access to class data

– Can be called without having a class instance

• This main function has just one statement

• We will cover static more later


Writing to the console• System.out.println("Hello world");

• System is an instance of a class that contains numerous predefined classes and methods

• out is an output stream class with methods print and println


print and println• Overloaded method names• Accept one parameter• There is a version for almost every

parameter type

Classes and objects• Sometimes these two terms are

used interchangably• A class is a type

– Defines possible values and actions

• An object is an instance of a class• Example

– A person name would be a class– “Curt Hill” would be an object, an

instance of a class


Finally• There are some other details that

need to be considered:– Handing in programs

• These will be covered in a subsequent presentation


copyright © 2004-2013 curt hill java looking at our first console application in eclipse

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